This invention relates to the field of semiconductor charge transfer devices and to the conveyance of signals between the control gate elements of such devices.
Charge coupled devices (CCD's) and Bucket Brigade devices (BBD's) are two forms of Charge transfer devices (CTD's). Charge coupled devices are frequently used for storing data, for transforming data from an optical image to an electrical signal and for reorganizing data from, for example, a serial format into a parallel format, and thence into a second serial format. Although parallel readout of the data stored in individual cells of a CCD array may be required in some applications, serial readout is also often required and in many instances is essential to use of the CCD concept. In the optical-to-electrical signal transducer application of a CCD, for example, an image may be impressed on an array of CCD's which are subsequently interrogated in some organized serial fashion in preparation for storing or processing the image related information.
Serial readout is a time-consuming operation involving numerous repetitions of a shift and read sequence, and in view of these numerous repetitions, there is great incentive to minimize the time required for each event included in a read cycle. Since CCD arrays capable of operation at frequencies of 1 gigahertz and above are now possible, read cycle event times in the order of fractions of nanosecond are required.
A major limiting consideration in the serial operation of CCD arrays in this speed range has concerned the currents required for driving the gate electrodes of the array and the circuitry and power consumption involved in the gate electrode drivers. For example, a CCD array which operates with 10 volt signal swings at the control gate, and has a total of 100 picofarads of gate capacitance, involves voltage change rates on the order of 20,000 volts per microsecond and current changes in the order of 4 amperes per nanosecond, or 2 amperes per half nanosecond with conventional driving of the array.
Because of these current levels and the resulting power dissipation in a gate driving circuit, it is common practice in such high-speed CCD arrays to locate the gate driving circuit external to the CCD microcircuit chip where the generated heat can be dissipated without effect on the performance or the size of the package required for the CCD array. The high speed current change however makes such coupling of drive signals from off chip to the CCD array difficult because the inductance of signal coupling bond wire is about 40 Nanohenrys per inch of length and the voltage developed across such as inductance is determined by the mathematical product of inductance and current rate of change (L di/dt). The expected current change rate of 4.times.10.sup.9 amps/second therefore produces a bond wire voltage drop of 1.6 volts per 1/100 inch of bondwire length--an unacceptable value in most circuit arrangements.
The present invention provides an alternative arrangement for driving the control gate electrodes and similar busses in large arrays and provides more reasonable and more easily attained driving current magnitudes. Use of the invention may in CCD arrays therefore allow consideration of returning the driving circuit to the microcircuit chip.
The invention involves use of inductive elements distributed in a particular configuration in a CCD array; distributed inductive elements have of course been used in the electrical arts for such diverse purposes as compensating for the resistive-capacitive rolloff in telephone lines, a use proposed by Oliver Heavyside, and for transmission line coupling between stages of a distributed amplifier in oscilloscope equipment, and for numerous other uses.
The patent art includes several examples of CTD gate electrode driving arrangements; these examples include the patent of Walter J. Butler et al, U.S. Pat. No. 3,819,954, which addresses the problem of signal level change occurring as a given input signal propagates through the stages of a CCD or BBD device. The Butler patent is principally concerned with the amplitude of the signal being propagated through the CTD stages.
Another example of the patent art relating to CTD devices is found in the patent of Takao Tsuchiya et al, U.S. Pat. No. 4,344,001 which concerns a clock driving circuit and a signal output circuit for a charge transfer device of either the BBD or the CCD type. The circuit invention of the Tsuchiya patent is concerned with the above-described problems of driving a clock bus or gate electrode bus in a CCD array and resolves these problems by way of providing an improved driving circuit.
Another example of CCD arrangements is found in the patent of Thomas E. Linnenbrink, U.S. Pat. No. 4,393,357 which is concerned with a CCD employed for high-speed data recording and involves the arrangement used for transferring charge between cells in a CCD. The Linnenbrink patent uses a first transmission line for coupling received signals into the charge transfer device structure and also for accomplishing the readout of stored signals. The Linnenbrink apparatus also uses electrostatic coupling between the transmission line and the charge device structure. Both the use of this electric field coupling between the line and charge device structures and the employment of the transmission lines for signals other than the cell-to-cell transfer clock distinguish the Linnenbrink apparatus from the present invention. Other differences in the Linnenbrink disclosure and the present invention include use of a "cutter gate" by Linnenbrink and the absence of large capacitance loading as provided by the control gate element capacitances in the Linnenbrink apparatus.